Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 192228
Area Forecast Discussion
National Weather Service Portland OR
228 PM PST Sun Feb 19 2017
.SYNOPSIS...An active weather pattern will bring periods of rain and
mountain snow to southwest Washington and northwest Oregon tonight
through Wednesday. A couple of shortwave troughs from the north will
move near or over the Pacific NW Wednesday night through next
weekend for cooler weather and a chance for low elevation snow.
.SHORT TERM...Today through Wednesday...Showers continue across SW
Washington and NW Oregon this afternoon and evening as a surface low
just off of the north Washington coast nears the B.C. coast. Isolated
thunderstorms are embedded within the showers offshore, and may move
inland this afternoon for brief heavy downbursts, gusty winds and
possibly small hail. Snow levels have risen to around 4000 ft to the
north and 5000 feet across Lane County. Showers have eased over the
Oregon Cascades, but will continue for the SW Washington Cascades,
where a winter weather advisory continues through 7 PM this evening.
The next low continues to be a challenge as the models vary greatly
on the strength and track of it. The forecast confidence for rain and
mountain snow is high with this low, but the confidence is much lower
for the forecast winds even though this low is within the 24 hour
forecast period. This low is developing well offshore the southern
Oregon coast this afternoon, but is not defined enough to be able to
tell if any of the models are initializing better than the others.
There also has not been a great ASCAT pass to compare forecast winds
Have decided to lean towards the GFS which forecasts strong winds
along the Oregon coast Monday afternoon and gusty south winds in the
Willamette Valley. The GFS shows the potential for warning level
winds along the coast, but will wait until the low becomes more
developed before completely jumping on the GFS band wagon. Especially
since the forecast winds are much lighter with the ECMWF model, and
located further north and inland with the NAM model.
The rain will become more persistent late tonight and Monday morning
as this low approaches the Central Oregon coast. Snow levels should
warm to above the Cascade passes by or with this more steady rain,
but the higher Cascades will see additional snow accumulation. Can
expect 0.5 to 0.75 inch of rain for the coast, coast range, and the
Cascades late tonight through Monday afternoon with around 0.5 inch
for the interior valleys. The Cascades between 4000 and 6000 feet
could see 2 to 5 inches of new snow.
Showers will ease Monday evening,and snow levels will lower to near
or just below the Cascade passes. Rain and Cascade snow will then
pick up again late Monday night as another low approaches from the
SW. Since the models are in poor agreement with the low expected
tonight, it is not too surprising that they are not in agreement on
this second low expected for Monday night. None of the model
solutions are forecasting very strong winds on Tuesday. Showers will
continue Tuesday and decrease Tuesday night.
An upper level shortwave trough then approaches from the north
Tuesday night, and will bring colder air across the Pacific NW.
Temperatures will likely cool into the 30s Tuesday night and remain
well below 50 degrees Wednesday afternoon. Snow levels will lower to
around 1500 feet with showers likely generating light accumulating
snow (1 to 3 inches) into the foothills and coast range on Wednesday.
.LONG TERM...Wednesday night through Sunday...An upper level ridge
over the NE Pacific will start to move the broad upper level trough
over the western conus to the east late next week. However, before
the trough moves inland, an upper level shortwave will drop south
from the Gulf of Alaska towards the Pacific NW Wednesday night and
Thursday. Models are in fair agreement that this low will move away
from the coast as it slides south which may limit moisture over
Oregon and Washington. Another shortwave trough will drop south from
the north this weekend. Colder air will result in low snow levels and
unseasonal cool temperatures Thursday through Sunday. Snow levels
will likely be below 1000 feet with a chance for showers. With snow
levels this low, can not rule out a few flurries making it close to
the valley floor, but limited moisture and warm road surface
temperatures will minimize the threat for accumulating snow and major
.AVIATION...Mostly VFR conditions this afternoon with widespread
showers. Have seen a few more lightning strikes near the coast
and could see a few more through the afternoon hours, but it`s
looking like any TS threat would remain near the coast. Expect
continued VFR conditions with some MVFR in heavier showers. Gusty
winds out of the S to SW will drop off between 00Z and 03Z then
turn more SE overnight. May see a break in showers this evening
but then steadier rain moves in from the south starting around
10Z Monday with possible MVFR conditions.
KPDX AND APPROACHES...-SHRA through this evening then steadier
rain with possible MVFR conditions after 12Z Monday. Gusty
southerly winds decrease and turn more SE/E after about 01Z.
.MARINE...The surface low which moves near the waters this
morning is now off the northern Washington coast but local winds
remain gusty to about 30 kt. Expect the gradient to weaken this
evening, with winds falling below 20 kt after about midnight.
Could still see some more thunderstorms across the waters through
the afternoon hours today and any thunderstorms would be capable
of producing higher wind gusts.
The active weather pattern continues as another potentially
strong low will affect the waters Mon. Despite being less than
24 hours away, confidence remains unusually low regarding the
strength, track, and therefore impacts of this low. However, it
is looking like the northern OR waters will not see gales. It`s
possible that area won`t even see gusts much above 20 kt, so have
not issued anything there for Monday. There is still enough of a
chance for gales across the waters off the central OR coast that
will keep the Gale Watch up. Not enough confidence to upgrade to
a Gale Warning, though, as only 1 of 3 main models shows gales.
Seas are currently around 10 to 12 ft and will be rising tonight
to 12 to 14 ft. The seas fcst for Mon is tough, given the
uncertainty in the surface low track, but the likeliest scenario
still appears to be seas peaking in the mid teens Monday
afternoon, highest near the central coast.
It look like another low of indeterminate strength will move onto
the southern OR/northern CA coast Monday night or Tuesday,
potentially bringing wind gusts up to about 30 kt to the central
and northern OR waters. Seas may drop below 10 ft Tuesday, but
may also stay above 10 ft until Tuesday night, depending on the
second low. After that, the pattern calms down for a few days
with more benign winds and seas potentially through the remainder
of the workweek. Bowen/MH
WA...Winter Weather Advisory until 7 PM PST this evening for South
PZ...Small Craft Advisory for winds until midnight PST tonight for
Coastal Waters from Cape Shoalwater WA to Florence OR out
Small Craft Advisory for hazardous seas until 7 AM PST Monday
for Coastal Waters from Cape Shoalwater WA to Florence OR
out 60 nm.
Gale Watch from Monday morning through Monday afternoon for
Coastal Waters from Cascade Head OR to Florence OR out 60
Small Craft Advisory for Rough Columbia River Bar from 10 PM
this evening to 2 PM PST Monday.
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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.